IV PREFACE. Some errors have been detected, and if many more should not be discovered by others, we shall be agreeably disappointed. We have, however, some confidence that they will not exceed in magnitude those which we have endeavored to eliminate from Conchological literature. All are liable to error: but it may be reasonably expected, that, as in Astronomy rude approxima¬ tions have been repeatedly corrected with successive advances towards absolute accuracy, so in Zoology the errors respecting geographical distribution and the characters of species will be gradually eliminated. A real advance towards accuracy, how¬ ever small, will be an ample reward for a labor which has been performed con amore. For other valuable assistance, especially in the preparation of Indices, I have'been indebted to Thomas Bland, Esq. Some of the sheets having been issued while the work was in .press, the dates at the bottom of the first page of each sheet may be taken for the dates of the publication of the new species. • i